1. Field of the Invention
The present invention relates to a technique for fabricating a complementary dielectrically isolated (DI) wafer structure and, more particularly, to a fabrication technique wherein the initial substrate material is altered to contain regions of both conductivity types.
2. Description of the Prior Art
For high voltage integrated device structures, some type of isolation between certain active regions is required to prevent premature breakdown of the device. Junction isolation, formed by including additional p-n junctions in the device structure, may be used for this purpose. However, these p-n junctions have a voltage limit themselves, and add to the over-all area occupied by the structure. As an alternative, dielectric isolation (DI) is used to surround the complete device with a layer of dielectric material. Many references exist in the prior art which describe this type of structure.
In many sophisticated circuit applications, it is required to provide both n-type and p-type devices on the same substrate (complementary structures). It is also useful in many applications to form separate regions with differing resistivities. Accomplishing these types of structures using dielectric isolation has been a formidable problem in the past. One solution is disclosed in U.S. Pat. No. 4,579,625 issued to A. Tabata et al. on Apr. 1, 1986. The method taught by Tabata et al. includes forming a plurality of projecting p-type polysilicon regions above a substrate surface, removing selected projections which are to be areas of n-type conductivity, and growing an n-type epitaxial layer over the entire surface of the device. The structure is then anisotropically etched to form n-type projections. The Tabata et al. process requires at least five photoresist operations, four silicon etches, and various other etching operations to form the complete structure.
An alternative fabrication technique is disclosed in U.S. Pat. No. 4,593,458 issued to M. S. Adler on June 10, 1986. Adler forms DI regions (tubs) of lightly-doped n-type conductivity and then selectively ion implants the various tubs to form either n-tubs or p-tubs. However, ion implantation as a doping method necessarily limits the type of final device formed to lateral devices only, since the diffusion gradient of the ion implanted species drops off at a rate such that the bottom area of the tub is rather lightly doped. Additionally, Adler discusses the formation of the epitaxial tub material directly on the conventional DI tub boundary of silicon dioxide. The process of forming an epitaxial region on top of a layer of silicon dioxide is not well understood, and is, at best, difficult to achieve.
U.S. Pat. No. 4,624,047 issued to S. Tani on Nov. 25, 1986 discloses yet another alternative method of forming complementary DI tubs. Tani replaces the projection-forming method of Tabata et al. with a method of forming p-type regions directly in an n-type substrate, through ion implantation, then processing with conventional DI processing to form the n-type and p-type tubs. As with the Adler structure, however, the use of ion implantation to form the p-type regions limits the downward diffusion of the dopant, restricting the final device structure to a latter form.
Therefore, a need remains for a complementary DI structure which is relatively easy to fabricate and allows for the formation of vertical, as well as lateral, device structures.